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T. A. Summers & E. S. Jaffe
CD25, weak staining for cyclin D1, and typically negative staining for CD5, CD23, and CD10. Caution is advised to always compare annexin A1 staining to that for a B-cell antigen, as annexin A1 is also present in myeloid cells and a proportion of T- cells. Annexin A1 is highly specific for HCL and can be used to differentiate HCL from SMZL and HCL- v, as annexin A1 is not expressed in any other B-cell lymphoma other than HCL [3]. Cyclin D1 expres- sion is not secondary to translocations involving the CCND1 gene. Interestingly, as in MCL, Sox11 may be expressed as well [4]. Enzyme cytochemistry for TRAP activity is infrequently done for diagnosis currently. Immunohistochemical detection of TRAP is feasible, but less specific; false negatives due to limited expression of the antigen are also a problem. HCL by flow cytometry commonly gates within
the monocyte region when analyzed by CD45 versus side scatter due to the characteristic surface irregula- rities of hairy cells. Additionally, this region is often devoid of actual monocytes. The classic phenotype of HCL is bright surface immunoglobulin, with bright co-expression of CD20, CD22, and CD11c. HCL also expresses CD103 and CD25, although these
markers are not HCL specific. It is nearly always negative for CD5. Spleens are enlarged macroscopically with expan-
sion of red pulp areas and zones of hemorrhage, which are often referred to as red blood cell lakes. The integrity of the cord-sinus architecture is disrupted, leading to pseudosinus formation in which sinusoidal-like spaces are lined by hairy cells with a loss of normal sinusoidal lining cells [5]. The HCL infiltrate is histologically similar to that described in the bone marrow and composed of small-to-medium sized lymphocytes with ample cytoplasm. White pulp areas are atrophic. Lymph nodes, when involved, are largely confined
to retroperitoneal and abdominal nodes, with rare involvement of peripheral nodes. The infiltrate dis- plays an interfollicular or paracortical distribution within the node, and may extend into surrounding adipose tissue. Sinuses are typically intact and follicular structures are spared. The cells have a fried-egg appearance, due to the abundant cytoplasm (Figure 3). The differential diagnosis of HCL in the bone marrow includes CLL, SMZL, and other forms of splenic lymphoma, including splenic diffuse red pulp small B-cell lymphoma and HCL-v. These latter two disorders are included in the 2008 WHO classification under splenic B-cell lymphoma/leuke- mia, unclassifiable (SBCL/L,u). The process known as splenic lymphoma with villous lymphocytes (SLVL) is probably heterogeneous and includes cases of SMZL and the disorders included under SBCL/L,u.
Figure 2. Hairy cell leukemia, bone marrow biopsy. (A) Bone marrow with relatively inconspicuous interstitial infiltrate of medium-sized lymphoid cells with ample cytoplasm (6200; H&E). (B) CD20 immunohistochemical stain aids in identification of neoplastic cells (6200; IHC).
Figure 3. Hairy cell leukemia involving lymph node. Replacement of lymph node paracortex and interfollicular areas by a relatively monotonous infiltrate of medium-sized lymphoid cells with ample cytoplasm. Sinuses are widely patent (640; H&E). Inset: high- power image of atypical infiltrate. The lymphoid cells have abundant cytoplasm and distinct cytoplasmic borders (6400; H&E).
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